Nest Thermostat suddenly saying no C wire power

[BlueOnBlue]

Hey all

So i bought this house in April and have had my new Nest thermostat installed since then, its been running happily since using the wiring that was existing (Y,G,R,W, and C)

LONG STORY SHORT  --- its been working just fine using the C wire to power... until this morning at 7am I get a notification from Google home saying the thermostat has lost power on the C wire and it would work better if it had power etc etc. 

Now two things - the rest of the house is powered just fine, and the AC seems to be working just fine since it's turning on and cooling the house to the set point.

The big problem here is this house is 3 hours away from me, since I bought it specifically to host as an AirBNB , and there are guests there right now until Friday.

I did ask the guest if he noticed any issues he said nope the AC is working fine no problems at all

So is this an error with the Nest where it says there is no power on the C wire but there in-fact is? I've been checking it remotely from Google Home and the message is still there " will work better with a C wire to avoid potential issues " and its been like that since the initial "power issue' at 704am this morning

I am checking the device information page often to see what the battery voltage displays, and it's still showing 3.01 volts since I checked it in the morning.. my assumption is the voltage would be going down if it was running on battery power only, until it gets to 0??

either way I plan to go there and see what's up on friday once the guests leave, so what should I be checking?

the only thing I can imagine is the breaker for the furnace tripped and that's why this is happening... but all the furnace does with the AC on is serve as a big fan unit to blow the air around. so no idea why it would have tripped when it was running happily since April 

[CoolingWizard · Accepted solution]

@BlueOnBlue you are correct on the actual operation of your system. If the transformer and the air handler is blown or the fuses blowing, then the air conditioner outside will not work either so there is a problem with the wire or possibly the Nest thermostat. Got an electrical spike That might have damaged the base plate.  

make sure you take yourself with a volt ohm meter and set the Meter to measure AC power. Then check between the R wire and the C wire and see if you see that there is 24 VAC +\- 4 volts.  First checked Volts between the R wire and the C wire at the thermostat, and see if you receive the voltage if you do not then go to your air handler open up the maintenance cover and check the red wire and the C wire on the terminal control board to see if it’s receiving 24 VAC there. 

if my answer helps you to understand the problem please check the kudos down below, and if my answer solved your problem, please click Recommend this Answer, below. 

AC Cooling Wizard

[BlueOnBlue]

Thanks , I don't have a volt meter but I may have to pick one up when I go there.

will update once I see what's going on!

[BlueOnBlue]

Alright, I just got back and here is the order of discovery:

1) First thing I did was check the voltage, I am very unfamiliar with using one but I think it was 30 VAC between the C wire and the R wire. I tried a bunch of different settings on the meter and the only one I could get a solid reading was the 30, and it was def. live because it was sparking when i touched it with the probes

2) I nudged the C wire to make sure it had good connection in the Nest terminal 

3) Put the Nest back on the plate and it cleared the error.

4) After all that and I confirmed it was working fine, I cut and stripped more of the C wire as some of the copper was exposed further along the wire outside the terminal block, so I just cut it and re-stripped it cleanly to prevent any future issues and ensured it was seated all the way inside the Nest terminal block

Right now after I left it was working fine and no more error message .. so possibility it was just 'loose' in the terminal?

Readings from the power menu :

Vin: 28.010 V

lin: 67 (W)

PS: Y-14, YW-78, W-67

And the battery menu:

Voltage 3.005

Impedance 978 mOhm

As before the AC unit is operating fine, the fans are blowing inside and the house maintains the temp set.

Thanks 

[CoolingWizard]

@BlueOnBlue when using the volt/ohm meter, set the meter AC Voltage. It might look like ~. 30 VAC is a bit high and the Nest Thermostat AC high input limit is 29VAC. 
The Nest Learning Thermostats have a 2-step connector. The first step tell the display there is a wire inserted.  The second step is the electrical contact point for that wire. If the wire is only at the first step, the button tab will come up slightly.  When the wire is fully inserted the button tab stays fully depressed.  

If my response helps to understand better, please tap Kudo thumb below. If my response solved your problem then please tap Recommend this Answer.  

AC Cooling Wizard

[BlueOnBlue]

I'm back!

So this mystery deepens more now, this is giving me a bigger headache

Last night at around 12 midnight, the thermostat went 'offline' once again, no warning messages or indications it just simply stopped communicating

** Once again as this is a remote property I am unable to physically be there to see it, and trust what I can see from the app

Since that happened last night the thermostat has not reconnected / come back online via the google home app.

I am assuming the batteries got to a low point and it turned off the wifi on the Nest unit, so this goes back to 'is the nest getting power over the R/C wire' ... it must be on and off causing the batteries to drain down for some reason.

My question now from all this is, IF the batteries are getting low and the thermostat turns off the WIFI , to save power... and then the R/C wire power is restored fully powering up the unit, DOES it turn the WIFI back on automatically or does it stay OFF, until someone goes into the settings and adjusts it

I ask because the guests that are currently staying there just arrived back, after being out all day. I expected them to call me and say what's going on its a sweat lodge in here! but they have not, so I rest assured the A/C is still working but again the WIFI connection the thermostat is turned off. 

My plan at this time is to head there Sunday when i can get the day, and probably pull a fresh thermostat wire,  re-do all the connections, and most likely install a Ecobee since the Nest has been giving me so much grief.

[CoolingWizard]

@BlueOnBlue 

The Nest only deactivated the wifi when the battery drops below 2.0 volts. The Nest should restore the wifi automatically when power reaches 3.0 volts again.  However sometimes it requires a reboot. To reboot the Nest you press and hold the display until the Google circle appears. 

AC Cooling Wizard